The present invention relates to a system for securing television broadcasts against unauthorized viewing. Specifically, a system for encoding a standard video signal in a plurality of scrambling modes is described.
Cable television, as well as direct satellite broadcasts, hotels, and other institutions, have developed signal distribution systems for delivering pay television broadcasts to various subscribers. These broadcasts can be carried out on several channels and different categories of programs can be supplied at a subscription rate different for each of these categories, or tiers of broadcast service.
Systems for scrambling the broadcast to prevent unauthorized viewing have been instituted by most of the program originators. Requirements to maintain a scrambling system cost to a minimum has necessarily limited the extent to which the signal can be scrambled. Consequently, unauthorized descrambling of the video signal is carried out with only a minimum of effort on the part of signal pirates.
An additional cost imposed upon scrambling broadcasts in a cable system include the necessity of most of these systems to send a serviceman to a subscriber's location to enable or disable reception of a pay television broadcast. It is therefore advantageous to provide a scrambling system which would permit enablement and disablement of subscriber decoders at the head end without the additional cost of a service person going to the subscriber's location.
One of the more popular techniques used for scrambling a television signal depends on altering the synchronization pulse of the base band video. These systems either suppress a group of synchronization pulses or otherwise modify the amplitude of the synchronization pulse, thus severely distorting the television signal viewed by an unauthorized subscriber. This earlier scrambling technique, however, is not as effective when received by the newer digital television receivers which are currently entering the marketplace. Those earlier scrambling techniques, which suppress horizontal synchronization pulses, are not as effective because the newer digital television receivers have a substantially greater horizontal stability. They need only to receive a minor number of correct horizontal sync pulses to still have sufficient information in which to generate a normal stable picture.
Video line inversion techniques have proven to be effective against these newer receivers. However, video line inversion, although providing a highly objectionable picture, nevertheless provides a picture not totally opaque. Consequently, for effective scrambling of the video signal, additional scrambling modes are required to provide total security against unauthorized viewing. This objective, however, must be carried out within the economic confines of keeping the cost of the scrambling system to the subscriber at a minimum.